Exfoliated nanocomposites diffusivity

When a solvent diffuses across a neat polymer, it must travel the thickness of the sample (do). When the same solvent diffuses through a nanocomposite film with nanoclays, its path length is increased by the distance it must travel around each clay layer it strikes. According to Lan et al. [99] the path length of a gas molecule diffusing through an exfoliated nanocomposite is... 

It is generally accepted that thermal stability of polymer nanocomposites is higher than that of pristine polymers, and that this gain is explained by the presence of anisotropic clay layers hindering diffusion of volatile products through the nanocomposite material. It is important to note that the exfoliated nanocomposites, prepared and investigated in this work, had much lower gas permeability in comparison with that of pristine unfilled PE [12], Thus, the study of purely thermal degradation process of PE nanocomposite seemed to be of interest in terms of estimation of the nanoclay barrier effects on thermal stability of polyolefin/clay nanocomposites. 

The dispersion of nano-sized fillers into polymer matrix creates a tortuous path for the diffusant in comparison to neat polymer having straight diffusive path perpendicular to the film (Figure 3). In case of intercalated and exfoliated nanocomposites, path travelled by diffusant molecule would be greater than that of aggregate micro composite. 

Figure 6. Dependence of water vapour diffusion log Do (Do = zero concentration diffusion coefficient) on the clay content of microcomposites, intercalated nanocomposites, and exfoliated nanocomposites.

These results make LDHs excellent candidates as fillers for polymer matrices, to be used as degradant additives, which permits a precise control of the lifetime of the plastic product, decreasing its negative impact on the environment. The improvement in the thermal properties of PE/LDH nanocomposites is usually explained assuming that the LDH layers act as a sort of barriers. Yue et al. [56] claim formation of nanostructures from the dispersed LDH particles, which decrease heat transfer, thus stabilizing the polymer chains. Dispersion of the LDH particles thus improves the barrier properties of the polymer nanocompounds because of their easy exfoliation. Fluid diffusion, especially gases, is strongly affected by this barrier effect. 

In the case of water vapour transport, the micro-composites as well as the intercalated nanocomposites show diffusion parameters very near to PCL, while the exfoliated nanocomposites strongly deviate showing much lower values, even at low montmorillonite content (Fig. 11.4). This is an indication that in the former cases the water molecules on specific sites are not immobilized but can jump fi om one site to another. Only in the case of the exfoliated samples, the inorganic platelets, dispersed in a disordered distribution, can constitute a barrier to the path of the hydrophilic molecules. 

Particularly interesting are the results on the MMT dispersion degree in the polymeric matrix. In the case of dichloromethane, for samples with 3 wt% of MMT it was shown that the diffusion parameter decreases going from microcomposites (values very similar to pure PCL) to exfoliated nanocomposites intermediate values of diffusion were measured for die intercalated nanocomposites. In the case of water, both micro-composites and intercalated nano-... 

Micro-composites as well as intercalated nanocomposites generally have diffusion parameters very near to the pure polymers, while exfoliated nanocomposites show much lower values, even at low silicate content. 

Having taken into account the above findings, it seems reasonable to explain the observed retardation of thermal oxidative degradation of st-PE-n-MMT by the capability of exfoliated MMT nanolayers to hinder the diffusion of oxygen throughout the partly cross-linked and carbonized nanocomposite matrix. 

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